Distributed fiber optic sensing systems are utilized in applications requiring architectural and structuring monitoring, temperature and pressure measurements and for intrusion detection. Standard single mode and multimode fibers are often used for these applications. Distributed fiber optic sensing systems have been utilized for monitoring key infrastructures such as buildings, oil pipelines, and bridges.
In infrastructure monitoring, distributed fiber optic sensing systems provide information on perturbations along the path that can trigger alarm when there is a substantial and/or abnormal change of fiber strain. Thus, distributed fiber optic sensing systems have important applications in ensuring the early detection of deterioration of these critical infrastructures and timely repairing when necessary.
In security applications, distributed fiber optic sensing systems provide information about an intrusion into a protected area or facility when a particular portion of perimeter is disturbed. The protected area or facility of high interest can be a government or military site, commercial airports, water treatment site, or power generation plant. The need for intrusion detection is greatly driven by the demand for securing sites carrying critical functions, and recently such need is further highlighted by the government's counter-terrorism agenda.
Distributed fiber optic sensing systems that utilize polarization effects require sensing a small change in the optical fiber. Such changes can be produced, for example, by stress, fiber bends, or changes in pressure. The state of polarization changes as light pulses travel along the fiber length, and is sensitive to fiber perturbation. The occurrence and location of a disturbance in the vicinity of a localized portion of the optical fiber can be determined based on the recognition that such a disturbance changes the polarization conditions of light backscattered from the disturbed portion of the optical fiber. The change of the state of the polarization in the optical fiber (sensing fiber) can be detected by the detection of the backscattered light utilizing a polarization-sensitive OTDR (POTDR) device. By inserting a polarization component such as a polarizer into the OTDR path, the polarization change can exhibit itself in the form of intensity change.
In general, because the signal-to noise ratio that can be extracted from the measured OTDR trace is low, monitoring techniques based on POTDR often require extensive filtering and averaging to produce a better signal to noise ratio.